Loudspeaker systems generally include an enclosure having an interior within which a driver is mounted in alignment with an opening formed in the front panel of an enclosure. In order to reinforce the low frequency output of the loudspeaker system, it is common to employ a port or a passive radiator in the speaker enclosure. Ports and passive radiators resonate at low frequencies, typically lower than that of the driver/enclosure system, and therefore enhance reproduction of low frequencies produced by the driver. In most cases, a ported or passive radiator speaker system has a wider low frequency bandwidth than a sealed-enclosure design.
A port is essentially a tube having a length dimension, or distance between its opposite ends, wherein one end is typically located at the front panel of the enclosure and the opposite end is positioned within the interior of the speaker enclosure. To maintain system linearity at very low frequencies, the cross sectional area of the port must be large. Further, a large cross sectional area is required when using ports to avoid “chuffing” or air flow noises. Ports with a large cross sectional area must be longer than small area ports in order to be tuned to the same low frequency. In applications where the speaker enclosure must be relatively small, such as subwoofer speaker systems designed for use in vehicles, it is not possible to use long, low-frequency ports due to space considerations. The speaker enclosure must house both the driver and the port, which requires a relatively large volume box. If the ports are designed with a smaller cross sectional area in an attempt to overcome the space issue, unacceptable air flow noises would result. On the other hand, tuning a port for higher frequencies, which allows it to be shortened and take up less space, sacrifices the desired system frequency response.
Passive radiators solve some of the issues with ports noted above because their length dimension may be much less than that of tubes and they may take up less volume in the interior of the speaker enclosure. One typical form of passive radiator resembles a driver, i.e. it includes upper and lower suspensions and a diaphragm, but the motor structure of the driver is eliminated. When mounted in a speaker enclosure, internal air pressure generated by axial movement of the driver creates air pressure waves that cause the passive radiator to move. Like ports, passive radiators resonate at a frequency lower than that of the driver/enclosure system. System linearity at very low frequencies is obtained with a passive radiator by making its radiating area, e.g. the surface of the diaphragm in the example given above, relatively large compared to that of the driver itself. While the length dimension of passive radiators may be made more compact than ports, the large radiating area requirement creates the need for a larger surface area at the baffle of the enclosure or the use of multiple enclosure faces. Such surface area may amount to the equivalent of two drivers or more since the radiating area of the passive radiator is larger than that of the diaphragm of the driver. Consequently, the overall dimension of the speaker enclosure needed when using a passive radiator may be greater than the area which is available in a given application. This is particularly true in the case of loudspeaker systems for use in vehicles wherein only one wall of the enclosure may be exposed for use as a radiating surface and/or if the total radiating area is small due to space constraints.